Name: Lau Shok Mun
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Saturday, 10 November 2012

Lunar eclipse

lunar eclipse occurs when the Moon passes directly behind the Earth into its umbra (shadow). This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, a lunar eclipse can only occur the night of a full moon. The type and length of an eclipsedepend upon the Moon's location relative to its orbital nodes. Unlike a solar eclipse, which can only be viewed from a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth. A lunar eclipse lasts for a few hours, whereas a total solar eclipse lasts for only a few minutes at any given place, due to the smaller size of the moon's shadow. Also unlike solar eclipses, lunar eclipses are safe to view without any eye protection or special precautions, as they are no brighter (indeed dimmer) than the full moon itself.

Types of lunar eclipse

Schematic diagram of the shadow cast by the Earth. Within the centralumbra shadow, the Moon is totally shielded from direct illumination by the Sun. In contrast, within thepenumbra shadow, only a portion of sunlight is blocked.
As seen by an observer on Earth on the imaginary celestial sphere, the Moon crosses the ecliptic every orbit at positions called nodes twice every month. When the full moon occurs in the same position at the node, a lunar eclipse can occur. These two nodes allow two to five eclipses per year, parted by approximately six months. (Note: Not drawn to scale. The Sun is much larger and farther away than the Moon.)
total penumbral lunar eclipsedims the moon in direct proportion to the area of the sun’s disk blocked by the earth. This comparison shows the southern shadow penumbral lunar eclipse of January 1999 (left) to the same moon outside of the shadow (right) demonstrates this subtle dimming.
The shadow of the Earth can be divided into two distinctive parts: the umbra and penumbra. Within the umbra, there is no direct solar radiation. However, as a result of the Sun’s large angular size, solar illumination is only partially blocked in the outer portion of the Earth’s shadow, which is given the name penumbra. A penumbral eclipseoccurs when the Moon passes through the Earth’s penumbra. The penumbra causes a subtle darkening of the Moon's surface. A special type of penumbral eclipse is a total penumbral eclipse, during which the Moon lies exclusively within the Earth’s penumbra. Total penumbral eclipses are rare, and when these occur, that portion of the Moon which is closest to the umbra can appear somewhat darker than the rest of the Moon.
partial lunar eclipse occurs when only a portion of the Moon enters the umbra. When the Moon travels completely into the Earth’s umbra, one observes a total lunar eclipse. The Moon’s speed through the shadow is about one kilometer per second (2,300 mph), and totality may last up to nearly 107 minutes. Nevertheless, the total time between the Moon’s first and last contact with the shadow is much longer, and could last up to 4 hours.[1] The relative distance of the Moon from the Earth at the time of an eclipse can affect the eclipse’s duration. In particular, when the Moon is near its apogee, the farthest point from the Earth in its orbit, its orbital speed is the slowest. The diameter of the umbra does not decrease appreciably within the changes in the orbital distance of the moon. Thus, a totally eclipsed Moon occurring near apogee will lengthen the duration of totality.
The timing of total lunar eclipses are determined by its contacts:[2]
P1 (First contact): Beginning of the penumbral eclipse. The Earth's penumbra touches the Moon's outer limb.
U1 (Second contact): Beginning of the partial eclipse. The Earth's umbra touches the Moon's outer limb.
U2 (Third contact): Beginning of the total eclipse. The Moon's surface is entirely within the Earth's umbra.
Greatest eclipse: The peak stage of the total eclipse. The Moon is at its closest to the center of the Earth's umbra.
U3 (Fourth contact): End of the total eclipse. The Moon's outer limb exits the Earth's umbra.
U4 (Fifth contact): End of the partial eclipse. The Earth's umbra leaves the Moon's surface.
P2 (Sixth contact): End of the penumbral eclipse. The Earth's shadow no longer makes any contact with the Moon.


selenelion or selenehelion occurs when both the Sun and the eclipsed Moon can be observed at the same time. This can only happen just before sunset or just after sunrise, and both bodies will appear just above the horizon at nearly opposite points in the sky. This arrangement has led to the phenomenon being referred to as a horizontal eclipse. There are typically a number of high ridges undergoing sunrise or sunset that can see it. Indeed, the reddened light that reaches the Moon comes from all the simultaneous sunrises and sunsets on the Earth. Although the Moon is in the Earth’s umbra, the Sun and the eclipsed Moon can both be seen at the same time because therefraction of light through the Earth’s atmosphere causes each of them to appear higher in the sky than their true geometric position.[3]
The Moon does not completely disappear as it passes through the umbra because of the refraction of sunlight by the Earth’s atmosphere into the shadow cone; if the Earth had no atmosphere, the Moon would be completely dark during an eclipse. The red coloring arises because sunlight reaching the Moon must pass through a long and dense layer of the Earth’s atmosphere, where it is scattered. Shorter wavelengths are more likely to be scattered by the air molecules and the small particles, and so by the time the light has passed through the atmosphere, the longer wavelengths dominate. This resulting light we perceive as red. This is the same effect that causes sunsets andsunrises to turn the sky a reddish color; an alternative way of considering the problem is to realize that, as viewed from the Moon, the Sun would appear to be setting (or rising) behind the Earth.
The amount of refracted light depends on the amount of dust or clouds in the atmosphere; this also controls how much light is scattered. In general, the dustier the atmosphere, the more that other wavelengths of light will be removed (compared to red light), leaving the resulting light a deeper red color. This causes the resulting coppery-red hue of the Moon to vary from one eclipse to the next. Volcanoes are notable for expelling large quantities of dust into the atmosphere, and a large eruption shortly before an eclipse can have a large effect on the resulting color.

Danjon scale

The following scale (the Danjon scale) was devised by André Danjon for rating the overall darkness of lunar eclipses:[4]
L=0: Very dark eclipse. Moon almost invisible, especially at mid-totality.
L=1: Dark eclipse, gray or brownish in coloration. Details distinguishable only with difficulty.
L=2: Deep red or rust-colored eclipse. Very dark central shadow, while outer edge of umbra is relatively bright.
L=3: Brick-red eclipse. Umbral shadow usually has a bright or yellow rim.
L=4: Very bright copper-red or orange eclipse. Umbral shadow is bluish and has a very bright rim.

Eclipse cycles

Every year there are at least two lunar eclipses, although total lunar eclipses are significantly less common. If one knows the date and time of an eclipse, it is possible to predict the occurrence of other eclipses using an eclipse cycle like the saros.

What causes a Lunar Eclipse ?
A lunar eclipse happens when the Moon passes through the Earth's shadow. Earth always has a shadow, which is created by the Sun. On those rare occasions when the Moon, Earth and the Sun are all lined up just right, the Moon passes through this shadow.This would happen every full moon if the Moon orbited around the Earth in the same plane as the Earth orbits around the Sun. The Moons orbit, however, is tilted about 5 degrees above the Earth-Sun plane. This tilt itself, however, rotates, allowing eclipses to happen when the tilt of this plane lines up with the Earth-Sun plane, blocking sunlight.
An eclipse of the Moon can only take place at Full Moon, and only if the Moon passes through some portion of the Earth's shadow. The shadow is actually composed of two cone-shaped components, one inside the other. The outer or penumbral shadow is a zone where some portion of the Sun's rays are blocked. In contrast, the inner or umbral shadow is a region devoid of all direct sunlight.
When can one view an eclipse ?
A lunar eclipse is visible over an entire hemisphere and is seen at the same time to everyone who is in sight of the full moon. Because of local time zones, however, the times of a lunar eclipse can span many hours.
How long does an eclipse last ?
Lunar eclipses can last for more than three hours because the Moon and the Earth are moving slowly in relation to each other, and the shadow cast by the Earth is so large. Because of their sizes and the relative distances between the Earth, Moon, and Sun, this shadow is much larger than that cast by the Moon on the Earth (during a solar eclipse).
Are all eclipses the same ?
Although eclipses are always caused by the same general lineup of Sun, Moon, and Earth, each lunar eclipse may have its own unique visual characteristic. Colors and the deepness of the shadow on the surface are affected by the type of eclipse, local weather conditions, atmospheric conditions, and the geographic location of the observer. When the Moon is in the darkest part of Earth's shadow, or in totality, it can have some beautiful colors, usually a dark pastel, such as violet or a very dark apricot.
What are the three types of eclipses?
  • Partial Lunar Eclipse
  • A portion of the Moon passes through the Earth's umbral shadow. These events are easy to see, even with the unaided eye.
  • Penumbral Lunar Eclipse
  • The Moon passes through the Earth's penumbral shadow. These events are subtle and quite difficult if not impossible to observe. During a penumbral eclipse the moons light is dimmed but does not go dark due to the fact that the penumbral shadow is not dark enough to black out the sun's light. A penumbral eclipse is sometimes referred to as an appulse eclipse.
  • Total Lunar Eclipse
  • The entire Moon passes through the Earth's umbral shadow. During the time of totality the moons color may change to a dull copper tone, an effect caused by earth shine or reflected earth light. The moon can stay in the umbrals shadow for as long as 90 minutes.

    Lunar eclipses occur when Earth's shadow blocks the sun’s light, which otherwise reflects off the moon. 
    The most recent lunar eclipse was Monday, June 4, 2012. The next lunar eclipse is Nov. 28, 2012. There are three types, with the most dramatic being a total lunar eclipse, in which Earth’s shadow completely covers the moon. Throughout history, eclipses have inspired awe and even fear, especially when total lunar eclipses turned the moon blood-red, an effect that terrified people who had no understanding of what causes an eclipse and therefore blamed the events on this god or that. Below, you’ll find the science and history of lunar eclipses, learn how they work, and see a list of the next ones on tap. [See also our guide to Solar Eclipses.]
    This montage of images taken by skywatcher Kieth Burns shows the Dec. 20, 2010 total lunar eclipse. The photos won a NASA contest to become an official NASA/JPL wallpaper for the public.
    CREDIT: NASA/JPL-via Kieth Burns

    What is a Lunar Eclipse?

    Because the moon’s orbit around Earth lies in a slightly different plane than Earth’s orbit around the sun, perfect alignment for an eclipse doesn’t occur at every full moon. A total lunar eclipse develops over time, typically a couple hours for the whole event. Here’s how it works: Earth casts two shadows that fall on the moon during a lunar eclipse: The umbra is a full, dark shadow. The penumbra is a partial outer shadow. The moon passes through these shadows in stages. The initial and final stages — when the moon is in the penumbral shadow — are not so noticeable, so the best part of an eclipse is during the middle of the event, when the moon is in the umbral shadow. [Lunar Eclipse Pictures | More Pictures | And More Pictures]
    Total eclipses are a freak of cosmic happenstance. Ever since the moon formed, about 4.5 billion years ago, it has been inching away from our planet (by about 1.6 inches, or 4 centimeters per year). The setup right now is perfect: the moon is at the perfect distance for Earth’s shadow to cover the moon totally, but just barely. Billions of years from now, that won’t be the case.
    Types of lunar eclipses
    • Total lunar eclipse: Earth’s full (umbral) shadow falls on the moon. The moon won’t completely disappear, but it will be cast in an eerie darkness that makes it easy to miss if you were not looking for the eclipse. Some sunlight passing through Earth’s atmosphere is scattered and refracted, or bent, and refocused on the moon, giving it a dim glow even during totality. If you were standing on the moon, looking back at the sun, you’d see the black disk of Earth blocking the entire sun, but you’d also see a ring of reflected light glowing around the edges of Earth — that’s the light that falls on the moon during a total lunar eclipse.
    • Partial lunar eclipse: Some eclipses are only partial. But even a total lunar eclipse goes through a partial phase on either side of totality. During the partial phase, the sun, Earth and moon are not quite perfectly aligned, and Earth’s shadow appears to take a bite out of the moon.
    • Penumbral lunar eclipse: This is the least interesting type of eclipse, because the moon is in Earth’s faint outer (penumbral) shadow. Unless you’re a seasoned skywatcher, you likely won’t notice the effect.
    The blood-red moon
    The moon may turn red or coppery colored during the total portion of an eclipse. The red moon is possible because while the moon is in total shadow, some light from the sun passes through Earth's atmosphere and is bent toward the moon. While other colors in the spectrum are blocked and scattered by Earth’s atmosphere, red light tends to make it through easier. The effect is to cast all the planet's sunrises and sunsets on the moon.
    The moon turned a blood red over the Sossusvlei Desert Lodge on NamibRand Nature Reserve in Namibia in this stunning photo taken by skywatcher George Tucker on June 15, 2011.
    CREDIT: George Tucker

    "The exact color that the moon appears depends on the amount of dust and clouds in the atmosphere," according to NASA scientists. "If there are extra particles in the atmosphere, from say a recent volcanic eruption, the moon will appear a darker shade of red."
    Christopher Columbus leveraged a blood-red eclipse in 1504 to frighten natives on Jamaica into feeding him and his crew. It was on Columbus’ fourth and final voyage to the New World. An epidemic of shipworms ate holes in the ships of his fleet; Columbus' was forced to abandon two ships. He then beached his last two on Jamaica on June 25, 1503. The natives welcomed the castaways and fed them. But after six months, Columbus’ crew mutinied, and robbed and murdered some of the Jamaicans, who had grown weary of feeding the crew.
    Columbus had an almanac that foretold a lunar eclipse on Feb. 29, 1504. He met the local chief, and told him the Christian god was angry with his people for no longer supplying food. Columbus said to expect a sign of God’s displeasure three nights later, when He would make the full moon appear "inflamed with wrath." When the blood-red moon came to pass, the natives were terrified and “with great howling and lamentation came running from every direction to the ships laden with provisions,” according to an account from Columbus’ son.
    Just before the total phase of the eclipse was about to end, Columbus said God had pardoned the natives and would bring the moon back. The crew was well fed until help arrived in November and Columbus and his men sailed back to Spain.

    When is the next lunar eclipse?
    Here is a schedule of upcoming lunar eclipses:
    Nov. 28, 2012: Penumbral Lunar Eclipse: The moon will not be in Earth’s full (umbral) shadow, but rather will pass through the faint (penumbral) shadow. Maximum occurs at 14:33 UT. Viewers will be hard-pressed to notice the shading. Visible from eastern Asia and Australia.
    April 25, 2013: Partial Lunar Eclipse: Visible from Europe, Africa, Asia and Australia.
    May 25, 2013: Penumbral Lunar Eclipse: Visible from the Americas and Africa.
    Oct. 18, 2013: Penumbral Lunar Eclipse: Visible from the Americas, Europe, Africa and Asia.
    April 15, 2014: Total Lunar Eclipse: Visible from the Americas, Australia and out in the Pacific Ocean.
    Oct. 8, 2014: Total Lunar Eclipse: Visible from the Americans, Asia, Australia and in the Pacific Ocean.
    This photo of the Dec. 20 total lunar eclipse by Jimmy Westlake shows the blue edge to Earth's shadow set against the reddened moon.
    CREDIT: Jimmy Westlake

    How to Watch a Lunar Eclipse
    Lunar eclipses are among the easiest skywatching events to observe. Simply go out, look up, and enjoy. You don’t need a telescope or any other special equipment. However, binoculars or a small telescope will bring out details in the lunar surface — moonwatching is as interesting during an eclipse as anytime. If the eclipse occurs during winter, bundle up if you plan to be out for the duration — an eclipse can take a couple hours to unfold. Bring warm drinks and blankets or chairs for comfort. [Lunar Eclipse Pictures | More Pictures | And More Pictures]